The invention relates to a device for measuring the mass flow of milk in particular during the milking process. The invention is generally suitable for determining the mass flow of milk from cows, ewes, goats, buffaloes, llamas, camels, dromedaries, or of other lactating mammals and will be described below exemplary with reference to the milking of cows.
In addition the invention may be used in other areas of application where the measuring of mass flow of foamy or foam-forming liquids is concerned such as measuring the mass flow of beers, soft drinks, fruit juices, or other similar food products, as well as foaming or foamy technical fluids, such as electroplating solutions.
Information about the current milk flow is advantageous for controlling the milking process so as to modify during milking e.g. the transition from the stimulating phase to the main milking phase, or to specify the removal time, or to modify other parameters during milking. Although a high accuracy level is usually not required, it is desirable.
Milk yield measuring is also significant for drawing conclusions about the performance of the individual cows. The milk yield can be gathered by way of integrating the mass flow of milk during milking. It is useful to have accuracies meet the requirements of ICAR since this would eliminate the need of regular separate milk yield measuring. Although the requirements for an ICAR (International Committee for Animal Recording) admission depend on the animal variety and other parameters, they are generally high. The allowable error in milk yield measuring for cows, assuming a milk yield larger than 10 kg, is 2 percent maximum at a standard deviation of 2.5%. As a rule, however, a general assessment of performance or controlling the milking process does not require this level of accuracy for determining the current mass flow.
One advantage of measuring the mass flow of milk is that in individual cases, the shape of the milk curve during milking or the total milk yield will allow to draw conclusions on the state of health of the animal.
One problem encountered in milk flow measuring is that milk is a heavily foaming fluid. Foam formation during milking is further intensified by the currently employed milking techniques since as a rule air is periodically or continually allowed to enter into the milk collection piece and/or the teat cups during milking to discharge the milk. Although the volume of air intake per unit time may vary, it will as a rule be approximately 8 liters of air/minute or even higher. Assuming a maximum milk flow e.g. for cows of approximately 10 or 12 liters of milk per minute in the main milking phase, the air volume to be discharged will roughly be at least approximately 25%, in particular at least 40% or even 50% of the volume flow of milk yield and air intake. And this quite considerable portion is already present during the maximum milk flow phase. Near the end of milking the proportion of air to be discharged will rise even higher due to the decreasing milk flow. Added to this there is the proportion of air entering at the teat cup due to less than tight sealing between the liner and the teat. This proportion can also be roughly estimated at e.g. 10 liters of air per minute. The considerable proportion of air in the air-fluid mixtures to be discharged may thus cause substantial formation of foam which considerably impairs milk flow measurements in flow.
Since the proportion of foam does not readily permit conclusions on mass from the volume, the accuracy of milk yield measuring methods through volumetric methods has its limits. Both the proportion of air in the fluid and the bubble size in the foam are not always even but they depend on a plurality of factors. These factors include, the milk temperature, the milk flow rate, the position and layout of the milk hoses, the type of milking unit, the type of teat liner, the milk hose diameter, the type of milking installation, the vacuum level and the pulse rate during milking, air leakages or air infiltration, the current state of health of the udder, individual differences between cows e.g. due to the lactation stage or the race of the cow, and due to differences in kind and quantity of feeding, etc.
Another problem in measuring the milk yield flow is caused by the periodic milk flow. Unlike measuring volumetric flow in many other applications, milk is drawn periodically. The teat space in the teat cup is subjected to a periodic vacuum such that milk will flow out of the teat approximately at the pulsation rate. The pulse rate typically lies between approximately 30 and 90 at e.g. 60 cycles per minute. Given four teats and identical rates with all of the teats, there will be a milk flow having approximately 60 milk flow pulses per minute. Where the udder halves or the four teats e.g. of a cow are selected variably, the high frequency proportion of the milk flow may increase to reach approximately 240 strokes per minute at a pulse rate of 60. Milk is often conveyed through the milk hoses in clusters such that short phases at maximum milk flow alternate with short phases at minimum milk flow. Determining the actual milk flow is difficult under these conditions.
Due to these influences, measuring the milk flow or the mass flow of milk is found to be difficult since the nature and composition of the foam phase on the one hand and on the other hand also the composition and quality of the liquid phase within one milking process and between milking processes are subject to fluctuations. For example the electrical conductivity of the fluid and the proportion of the foam phase may vary continually since e.g. the fat content may change during milking which will cause fluctuations in terms of electrical conductivity and the optical properties of the milk. Measuring methods based on measuring these parameters may thus be subjected to considerable inaccuracies.
Instead of measuring mass flow through volumetry, measuring methods and measuring devices have thus become known which take the fluid density into account. To this end, the electrical, optical, or e.g. acoustic impedance in flow can be measured.
EP 0 536 080 A2 discloses milk flow measuring in flow wherein the milk is guided through flow channels, and transmission of an infrared light beam through the milk is measured and analyzed. The temporarily dampened or dimmed infrared light beam through the channel as milk is flowing through allows to draw conclusions on the momentary mass flow of milk through said channel. One drawback of optical measuring is for example that small and large foam bubbles can scatter the light beam employed for measuring such that in the presence of foam not enough light can be measured in transmission or reflection measuring so as to result in measuring errors.
DE 37 37 607 A1 discloses another method and a device for milk flow measuring in flow. A plurality of electrodes positioned one above the other is provided to firstly determine the electrical impedance or electrical conductivity of the liquid-air mixture on the respective levels by means of the electrodes. In the bottom region, a reference conductance of the liquid currently passing is measured. On the basis of each height value the stepped level profile of the specific impedance is calculated by means of the reference conductance. The flow velocity of the draining off liquid is known for known impedance profiles from calibration measuring such that the stepped level profile permits conclusions on the flowing mass of the milk. Due to the principle applied, this known device is very complicated in terms of mechanics and electronics.
Another basic problem in measuring the mass flow of a foaming fluid by way of such a density profile or a fill height is that foam bubbles may accumulate and remain stuck in the measuring area without the foamy portion draining off. Such stationary foam may result in measuring errors.
For example when stationary foam is present in the measuring area and then the electrical impedance between two electrodes is measured or the optical impedance between transmitter and receiver or the acoustic dampening or the like, to determine the fill height or the level profile of density in front of a diaphragm, the stationary foam portion will be included in each measuring. A thus measured fill height is higher than the fill height of the actually flowing fluid. Referring back to the calibration parameter with this fill height will thus provide a faulty mass flow. In this example the mass flow value measured is higher than the actually flowing mass since the actually measured value is higher by the accumulated and in this example stationary foam portion.
Another measuring error results from foam accumulating in a quantity such that the stationary foam is pressing on the flowing fluid, thus increasing the flow velocity. Consequently the assumed flow velocity no longer corresponds to the calibrated flow velocity, and measuring the mass flow will be faulty. Such faults may occur in particular where a large proportion of the clear flow cross-section is filled with foam.
It has been known in the prior art to prevent measuring inaccuracies caused by stationary foam pressing against the measured fluid by positioning an expansion chamber in front of the measuring area to receive large amounts of foam. However, this method cannot prevent the weight of the stationary foam from accelerating the flowing fluid. Moreover, devices thus equipped are relatively large in structure. Also, even a large expansion chamber can entirely fill up with foam. Then, cleaning the foam expansion chamber may prove difficult.
For the given reasons it should be reliably prevented at least during a large part of the main milking phase that the foam portion accumulate in the measuring area and that stationary foam press on the liquid phase, so as to obtain sufficiently accurate measuring results.